U.S. patent application number 12/880313 was filed with the patent office on 2010-12-30 for load-balancing cluster.
Invention is credited to David Fullagar, Jeffrey Koller, Christopher Newton, Maksim Yevmenkin.
Application Number | 20100332664 12/880313 |
Document ID | / |
Family ID | 45831926 |
Filed Date | 2010-12-30 |
United States Patent
Application |
20100332664 |
Kind Code |
A1 |
Yevmenkin; Maksim ; et
al. |
December 30, 2010 |
LOAD-BALANCING CLUSTER
Abstract
A load-balancing cluster includes a switch having a plurality of
ports; and a plurality of servers connected to at least some of the
plurality of ports of the switch. Each server is addressable by the
same virtual Internet Protocol (VIP) address. Each server in the
cluster has a mechanism constructed and adapted to respond to
connection requests at the VIP by selecting one of the plurality of
servers to handle that connection, wherein the selecting is based,
at least in part, on a given function of information used to
request the connection; and a firewall mechanism constructed and
adapted to accept all requests for the VIP address for a particular
connection only on the server that has been selected to handle that
particular connection. The selected server determines whether it is
responsible for the request and may hand it off to another cluster
member.
Inventors: |
Yevmenkin; Maksim; (Thousand
Oaks, CA) ; Fullagar; David; (Boulder, CO) ;
Newton; Christopher; (Thousand Oaks, CA) ; Koller;
Jeffrey; (Oxnard, CA) |
Correspondence
Address: |
LEVEL 3 COMMUNICATIONS, LLC;c/o CPA Global
P.O. Box 52050
Minneapolis
MN
55402
US
|
Family ID: |
45831926 |
Appl. No.: |
12/880313 |
Filed: |
September 13, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12390560 |
Feb 23, 2009 |
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12880313 |
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61064339 |
Feb 28, 2008 |
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Current U.S.
Class: |
709/227 |
Current CPC
Class: |
H04L 61/2007 20130101;
H04L 29/12405 20130101; H04L 67/1002 20130101; H04L 67/101
20130101; H04L 67/2842 20130101; H04L 67/1036 20130101; H04L 67/02
20130101; H04L 69/326 20130101; H04L 67/1023 20130101; H04L 67/1014
20130101; H04L 67/141 20130101; H04L 61/2528 20130101; H04L 67/1068
20130101 |
Class at
Publication: |
709/227 |
International
Class: |
G06F 15/16 20060101
G06F015/16 |
Claims
1. A load-balancing cluster comprising: (A) a switch having a
plurality of ports; and (B) a plurality of servers connected to at
least some of the plurality of ports of the switch, each of said
servers being addressable by the same virtual Internet Protocol
(VIP) address, wherein each of said plurality of servers comprises:
a mechanism constructed and adapted to respond to a connection
request at the VIP address by selecting one of said plurality of
servers to handle that connection, wherein said selecting is based,
at least in part, on a first given function of information used to
request the connection; a firewall mechanism constructed and
adapted to accept all requests for the VIP address for a particular
connection only on the server that has been selected to handle that
particular connection; a mechanism constructed and adapted to
determine, when a server has been selected and when a request for a
resource has been made, whether the server is responsible for the
request for the resource, said determining being based, at least in
part, on information associated with the request for the resource;
and a handoff mechanism constructed and adapted to handoff a
network connection to another of said plurality of servers and to
accept a handoff request from another of said plurality of servers,
wherein the handoff mechanism determines whether to handoff a
network connection or to accept a handoff request based on at least
one attribute associated with the requested resource.
2. A load-balancing cluster as recited in claim 1, wherein the at
least one attribute associated with the requested resource
comprises at least one of the size of the requested resource and
the popularity of the requested resource.
3. A load-balancing cluster as recited in claim 2, wherein the
handoff mechanism is configured to accept a handoff request if the
size of the requested resource exceeds a threshold value.
4. A load-balancing cluster as recited in claim 2, wherein the
handoff mechanism is configured to accept a handoff request if the
popularity of the requested resource does not exceed a threshold
value.
5. A load-balancing cluster as recited in claim 2, wherein the
other of said plurality of servers that does have a copy of the
requested resource is configured to reject the handoff request if
the popularity of the requested resource exceeds a threshold value,
and wherein in response to receiving the rejection, the server
obtains and serves the requested resource.
6. A load-balancing cluster as recited in claim 2, wherein the
handoff mechanism is configured to accept a handoff request if the
popularity of the requested resource does not exceed a popularity
threshold value and the size of the requested resource exceeds a
size threshold value.
7. A load-balancing cluster as recited in claim 1 wherein said
request for the resource is an HTTP request.
8. A load-balancing cluster as recited in claim 7 wherein the
information associated with the request for the resource comprises
a URL.
9. A load-balancing cluster as recited in claim 8 wherein the HTTP
request is an HTTP GET request and wherein the information
associated with the request for the resource comprises a URL and at
least one HTTP header.
10. A method, operable in a load-balancing cluster comprising: a
switch having a plurality of ports; and a plurality of servers
connected to at least some of the plurality of ports of the switch,
each of the plurality of servers being addressable by the same
Internet Protocol (IP) address, and each of the plurality of
servers having a unique hardware address, the method comprising:
obtaining a connection request at the cluster to connect to a
server associated with the IP address, wherein the connection
request is for a resource; providing the connection request to each
server connected to the switch; at least one of the plurality of
servers determining which of the plurality servers is to handle the
connection, wherein the determining act is based, at least in part,
on a given function of information used to request the connection;
and if a first server of the plurality of servers that is
determined to handle the connection does not have a copy of the
requested resource, the first server providing a notification to a
second server of the plurality of servers that does have a copy of
the requested resource, wherein the notification indicates that the
first server does not have a copy of the requested resource.
11. A method as recited in claim 10, further comprising:
determining, by the second server, whether to: i) provide a copy of
the requested resource to said server, or ii) request the server to
handoff the connection to the second server so that the second
server can serve the requested resource, wherein the determining
act performed by the second server is based on an attribute of the
requested resource.
12. A method as recited in claim 11, wherein the attribute of the
requested resource comprises at least one of a size of the
requested resource and a popularity of the requested resource.
13. A method as recited in claim 10, wherein the notification is a
hand-off request.
14. A method as recited in claim 10, wherein the notification is a
peer-fill request.
15. A method as recited in claim 11, further comprising: the second
server providing a copy of the requested resource to the first
server if at least one of: the size of the requested resource does
not exceed a size threshold, and a popularity value associated with
the requested resource exceeds a popularity threshold.
16. A method as recited in claim 15, further comprising: upon
receiving at least a portion of the requested resource from the
second server, the first server serving the requested resource.
17. A method as recited in claim 11, further comprising: the second
server requesting the first server to handoff the connection to the
second server if at least one of: the size of the requested
resource exceeds a size threshold, and a popularity value
associated with the requested resource does not exceed a popularity
threshold.
18. A method as recited in claim 17, further comprising: the first
server handing off the connection to the second server; and the
second server serving the requested resource.
19. A method as in claim 10 wherein the given function comprises a
hash function.
20. A method as in claim 10 wherein the information used by the
given function comprises: the VIP, port information, the number of
servers in the cluster, the number of serving servers in the
cluster, and a server number.
21. A method as in claim 10 wherein the hardware address is a Media
Access Control (MAC) address and wherein the connection is a
Transmission Control Protocol (TCP) connection.
22. A method for load-balancing across a plurality of servers
connected to a switch, wherein each of the plurality of servers is
addressable by the same Internet Protocol (IP) address, the method
comprising: receiving a connection request to connect to a server
associated with the IP address, wherein the connection request is
for a resource; determining which of the plurality servers is to
handle the connection; if a first server of the plurality of
servers that is determined to handle the connection does not have a
copy of the requested resource, the first server providing a
notification to a second server of the plurality of servers that
does have a copy of the requested resource, wherein the
notification indicates that the first server does not have a copy
of the requested resource; and in response to receiving the
notification from the first server, determining, by the second
server, whether to: i) provide a copy of the requested resource to
said server, or ii) request the server to handoff the connection to
the second server so that the second server can serve the requested
resource, wherein the determining act performed by the second
server is based on an attribute of the requested resource.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part (CIP) of and
claims priority under 35 U.S.C. .sctn.120 to U.S. patent
application Ser. No. 12/390,560, filed Feb. 23, 2009, titled
"Load-Balancing Cluster," the entire contents of which are
incorporated herein by reference for all purposes. Application Ser.
No. 12/390,560 is related to and claims priority under 35 U.S.C.
.sctn.119(e) to U.S. Patent Application No. 61/064,339, filed Feb.
28, 2008, titled "Load-Balancing Cluster," the entire contents of
which are incorporated herein by reference for all purposes.
FIELD OF THE DISCLOSURE
[0002] This invention relates to content delivery.
GLOSSARY
[0003] As used herein, unless stated otherwise, the following terms
or abbreviations have the following meanings: [0004] MAC means
Media Access Control; [0005] MAC address means Media Access Control
address; [0006] IP means Internet Protocol; [0007] TCP means
Transmission Control Protocol; [0008] "IP address" means an address
used in the Internet Protocol to identify electronic devices such
as servers and the like; [0009] ARP means Address Resolution
Protocol; [0010] HTTP means Hyper Text Transfer Protocol; [0011]
URL means Uniform Resource Locator; [0012] IGMP means Internet
Group Management Protocol; [0013] DNS means Domain Name System.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The following description, given with respect to the
attached drawings, may be better understood with reference to the
non-limiting examples of the drawings, wherein:
[0015] FIG. 1 depicts a load-balancing cluster; and
[0016] FIG. 2 depicts an exemplary TCP connection handoff; and
[0017] FIGS. 3-4 are flowcharts of a TCP connection handoff.
[0018] FIG. 5 depicts a collection of load-balancing clusters.
[0019] FIG. 6 is a flowchart of processing associated with server
interactions.
[0020] FIG. 7 is a flowchart of processing associated with server
interactions.
THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS
[0021] As shown in FIG. 1, a load-balancing cluster 10 is formed by
an n-port switch 12 connected to a number (between 1 and n) of
servers 14-1, 14-2, . . . , 14-m, where m.ltoreq.n (collectively
"servers 14") via ports 16-1, 16-2, . . . , 16-n. Not every port
16-k of the switch 12 needs to have an actual (or operating) server
14 connected thereto. The switch 12 is preferably an Ethernet
switch.
[0022] Each server 14-j includes a processor (or collection of
processors) constructed and adapted to provide data in response to
requests. In presently preferred implementations, all servers are
the same and run the same version of operating system (OS), with
same kernel and software. However, those skilled in the art will
realize and understand, upon reading this description, that the
servers may be any server running any type of server processes.
Those skilled in the art will further realize and understand, upon
reading this description, that the servers need not all be the
homogeneous, and heterogeneous servers are contemplated herein.
[0023] Each server 14-j in the cluster 10 is addressable by a
unique hardware address--in the case of the Ethernet, a so-called a
MAC address (also known sometimes as an Ethernet address). For the
purposes of this description, the MAC or actual hardware address of
the j-th cluster server is denoted MACj.
[0024] The servers 14 in the load-balancing cluster 10 are all
assigned the same virtual IP address (VIP), e.g., "10.0.0.1". Each
server preferably also has at least one other unique (preferably
local) IP address, denoted IPj for the j-th cluster server.
Preferably a VIP address is also has MAC address (denoted MACVIP)
associated with it, with the VIP's MAC address being shared by all
the servers in a cluster. That is, in preferred embodiments, the
(VIP, VIP's MAC address) pair, i.e., (VIP, MACVIP) is the same for
all the servers in a cluster. However, as noted, each server also
preferably has its own private (IP address, IP's MAC address) pair
(e.g., (IPi, MACi)).
[0025] The servers 14 in cluster 10 are addressable externally
(e.g., from network 17, e.g., the Internet) via the local
(Ethernet) network 13 and switch 12. For example, using router 11,
an external request from client 19 via network 17 (such as the
Internet) to the IP address VIP is directed via the switch 12 to
each real cluster server 14-j connected thereto. The switch 12
forwards Ethernet frames, preferably as fast and as efficiently as
possible. The switch 12 may perform one-to-one (unicast) forwarding
or one-to-many (broadcast or multicast) forwarding. In unicast
forwarding a packet enters the switch on one port and leaves the
switch on another port. In the case of broadcast or multicast
forwarding packet enters the switch on one port and multiple copies
of the same packet leave the switch on many ports. When broadcast
forwarding (using, e.g., a so-called "unlearned" unicast MAC
address), the switch sends all incoming packets to every port,
whereas when multicasting mode (using a multicast MAC address), the
switch sends all packets to those ports that have servers connected
thereto.
[0026] In either case, the desired result is that all cluster
members--i.e., all servers 14 connected to the switch 12--get all
packets destined for the IP address VIP.
[0027] In case of multicast MAC address, the switch 12 may use
so-called "IGMP snooping" to learn which physical ports belong to
live servers. In case of an "unlearned" unicast MAC address, the
switch 12 forwards incoming traffic to all ports.
[0028] The system is not limited by the manner in which the switch
12 provides packets to the servers 14 connected thereto. Those
skilled in the art will realize and understand, upon reading this
description, that different and/or other methods of achieving this
result may be used.
[0029] In a local Ethernet network, an Ethernet MAC address is used
to identify a particular host machine connected to the network. In
such a network, a protocol such as, e.g., ARP, may be used to
translate between a host's IP address and its Ethernet MAC address.
For example, a host on an IP network wishing to obtain a physical
address broadcasts an ARP request onto the IP network. A host on
the network that has the IP address in the request then replies
with its physical hardware address.
[0030] An IP router provides a gateway between two (or more) IP
networks. The purpose of an IP router is to forward IP packets from
one IP network to another. An IP router should have an interface
and IP address in each network to which it is connected. So, IP
router 11 has at least two interfaces and two IP addresses: one IP
address to connect to the upstream network (17 in FIG. 1) and the
other IP address to connect to the local Ethernet network (13 in
FIG. 1).
[0031] A request from client 19 is made to the IP address VIP (via
network 17) and reaches the router 11. The request comes into the
router 11 via the interface connected to the upstream network 17,
and the router 11 forwards the request to the VIP (on the local
Ethernet network 13).
[0032] Because the local network 13 is an Ethernet network and
because router 11 is connected directly to the local network 13,
the router 11 encapsulates the IP packet (i.e., the request) into
an Ethernet packet before sending it. In order for the router 11 to
know where to send the Ethernet packet, the router makes an ARP
request. Once the Ethernet packet is sent, the switch 12 forwards
it to the server(s) 14.
[0033] In order to affect ARP mapping, a router (e.g., router 11)
typically maintains a so-called ARP table 15 (mapping IP addresses
to the MAC addresses of hosts connected thereto). In this manner,
when an IP packet is sent to a particular host that is connected to
the router 11, the router automatically resolves to the destination
host's MAC address and forwards the packet to the appropriate host.
The router 11 will try to deliver the IP packet directly to
destination (i.e., the VIP) because the router is connected to the
same local Ethernet network.
[0034] Certain special MAC addresses (e.g., broadcast or multicast)
can be used to instruct a switch to broadcast (or multicast) a
packet, thereby providing a packet to all hosts connected to that
switch. Specifically, e.g., an Ethernet switch sends a packet with
a broadcast or multicast MAC address in its destination field to
every port (or every port with a server connected thereto), whereby
every host/server connected to the Ethernet switch should get a
copy of the packet.
[0035] In order for two machines (e.g., client 19 and one of the
servers 14) to interact, a network connection must be established
between them. The client 19 has the IP address of a server (in this
case VIP), and tries to establish a connection via the network 17
and the router 11.
[0036] When the router 11 gets a request to connect to a server
with the IP address VIP (shared by the cluster servers 14-j), the
router maps the IP address VIP to a special MAC address that causes
the switch 12 to forward the request to each server connected
thereto. In the case of the load-balancing cluster 10, preferably
the switch 12 treats the MAC address for a VIP as a multicast
Ethernet address. Consequently, each member of the cluster 12
(i.e., each server 14) sees all incoming traffic (addressed to
VIP). The router's ARP table 15 thus gets a multicast Ethernet
address for the VIP, and thus, at the IP layer, all incoming
traffic to the VIP address is provided to all servers 14 connected
to the switch 12.
[0037] In a presently preferred implementation, the switch 12
maintains a so-called "forwarding database," (FDB 23 in FIG. 1) to
map destination Ethernet MAC addresses to physical Ethernet ports
16 on switch 12. When switch 12 receives an Ethernet packet, the
switch queries the forwarding database (e.g., using the destination
MAC address as a key) and tries determine which physical port
should be used to send the Ethernet packet out. This forwarding
database 23 allows switch 12 to forward Ethernet packets only where
they should go.
[0038] However, when switch 12 receives an Ethernet packet and
cannot find an entry in its forwarding database for a destination
Ethernet MAC address (i.e., e.g., in the case of an
unknown/unlearned MAC address), the switch forwards such an
Ethernet packet to all the ports (except the one it came from).
[0039] A multicast Ethernet MAC address has entry in the switch's
12 forwarding database instructing it to forward Ethernet packet to
multiple ports 16.
[0040] An Ethernet switch will generally try to learn by looking at
the MAC addresses of all the Ethernet packets passed through the
switch and will try to update its forwarding database accordingly.
However, it is preferable to ensure that the switch 12 never
"learns" about MAC address for the VIP and never builds an
association between VIP cluster MAC addresses and physical ports
16. The switch 12 is thereby forced to always forward Ethernet
packets destined for the cluster MAC address (and thus the cluster
VIP) to multiple/all ports 16.
[0041] Those skilled in the art will realize and understand, upon
reading this description, that different and/or other ways of
causing the switch to provide incoming data to all cluster members
may be used.
[0042] Having found a cluster server with the IP address VIP, a TCP
connection must be established between the client 19 and that
cluster server 14. A TCP connection is established between two
machines, in part, using a well-known three-way handshake (SYN,
SYN/ACK, ACK). This protocol is described, e.g., in "RFC
793--Transmission Control Protocol," September 1991, the entire
contents of which are incorporated herein by reference for all
purposes.
[0043] In the cluster 10, when a TCP connection is first
established, each cluster member (i.e., each server 14) effectively
decides which server 14 will handle a connection. In effect, each
cluster member decides for itself whether or not to handle a
connection. Once a particular cluster member takes (or is given)
responsibility for a particular connection, the other cluster
members do not handle (and need not even see) traffic related to
that connection. The manner of server selection is described
below.
[0044] Each cluster member (server) includes a stateful firewall
(FW) mechanism that is used to filter unwanted incoming traffic. In
FIG. 1, for the purposes of this discussion, the firewall mechanism
for the j-th server is denoted 20-j. Upon receipt of an IP packet,
the firewall first determines whether the packet is for an old
(i.e., already established) connection or for a new connection. For
already-established connections each firewall mechanism is
configured to reject incoming traffic that does not have an entry
in its firewall state table 22, and only to accept incoming traffic
that does have an entry in its firewall state table. In FIG. 1, the
firewall table for the j-th server is denoted 22-j. The firewall
must still inspect packets associated with new connections (i.e.,
connections in the process of being established, specifically
packets with only SYN flag set). To summarize: first the firewalls
make a decision as to whether an IP packet is "new" or "old". If
the packet is "old" then it is discarded unless a state entry
exists. If the packet is "new" it is passed for further inspection
(e.g., load balancing) and then, depending on the results, can be
either discarded or accepted.
[0045] Once it is determined (e.g., as described below) that a
particular cluster member 14-j is going to handle incoming traffic
on a certain connection, a corresponding entry is created in that
member's firewall state table 22-j. Specifically, the cluster
member/server creates a firewall state table entry for any packet
that belongs to a connection initiated from or accepted by the
cluster member. If a packet indicates that a remote host wishes to
open a new connection (e.g., via an IP SYN packet), then such
packet gets inspected by a firewall rule that determines whether or
not the cluster member should accept it. If the packet was accepted
by a cluster member, the firewall state table for that cluster
member is updated and all subsequent packets on the connection will
be accepted by the cluster member. The firewalls of the other
cluster members will block packets that they are not supposed to be
processing (i.e., packets that do not belong to connections they
initiated or accepted).
[0046] The firewall rule preferably ensures that only one cluster
member will accept a particular connection, however in some cases,
it is possible that more than one cluster member decide to accept
the same connection. This situation would create duplicate
responses from the cluster. However, as those skilled in the art
will realize and understand, upon reading this description, this is
not a problem for a TCP connection because the remote host will
only accept one response and discard others. In this scenario only
one cluster member will be able to communicate with the remote
host, other cluster members will have a stuck connection that will
be closed due to timeout. In the case when no servers respond to an
initial SYN packet the client will retry and will send another SYN
packet after a timeout. While cluster members may have inconsistent
state, they should converge and achieve consistent state
quickly.
[0047] The firewall determines which cluster member should handle a
particular connection using a given mapping function, preferably a
hash function. By way of example, the hash function jhash, a
standard hash function supplied in the Linux kernel, may be used.
Those skilled in the art know how to produce a number in a
particular range from the output of a hash function such as jhash.
The hash function produces an integer value. To obtain a value in
the range 1 to m, for some m, the output of the hash function is
divided by m and the remainder is used (this operation may be
performed using an integer remainder or modulo operation). For load
balancing in a cluster, the value of m is the number of currently
live servers in the cluster. Those skilled in the art will realize
and understand, upon reading this description, that the function's
output value need not be offset by one if the buckets are numbered
starting at zero.
[0048] Using, e.g., jhash, the function MAP(source IP, m) may be
implemented as:
(jhash(parameters)modulo m)
[0049] If there are m alive servers in a cluster, each server 14
performs the (same) mapping function (with the same inputs). Each
server or cluster member 14 is associated with a particular local
server number (or agent identifier (ID)). E.g., if there are eight
servers 14-0, . . . , 14-7, their corresponding agent IDs may be 0,
2, . . . , 7, respectively. Each server compares the result of the
mapping function (e.g., hash modulo m) to its local server number.
If the result of the mapping function is equal to the local server
number, the packet is accepted, otherwise the packet is
dropped.
[0050] Note that the exemplary functions shown above all operate on
values related to the particular connection (e.g., source and
destination address and port information). However, in a simplified
case, the mapping function may be one which merely takes as input
the number of active servers (MAP (m).fwdarw.{1 . . . m}). An
example of such a function is a round-robin function. Another
example of such a function is one which uses external (possibly
random) information. Note, however, that since all servers have to
use the same mapping function and have to produce the same result,
such a function would need to access a global space and all
invocations of such a function (from each cluster server) would
need to be operating on the same values.
Example I
[0051] By way of example, and without limitation, consider a
cluster with 8 ports and with 7 active servers connected to those
ports as shown in the following table:
TABLE-US-00001 Port #. 0 1 2 3 4 5 6 7 Server S0 S1 S2 S3 S4 -- S6
S7 Bucket 0 1 2 3 4 5 6
[0052] In this case, the number of active servers, m, is 7, there
are seven buckets (numbered 0 to 6), and so the mapping function
should produce a number in the range 0 to 6. Suppose, for the sake
of this example, that the mapping function is:
MAP(source IP,destination IP,destination port, m)=hash(source
IP,destination IP,destination port)modulo m
[0053] If a connection request comes in from IP address
123.156.189.123, for the VIP (1.0.0.1) on port 80. Each server runs
the mapping function:
hash(123.222.189.123,1.0.0.1,80)modulo 7
[0054] Suppose that this mapping produces a value of 4 then server
S4 (which corresponds to bucket 4) handles the connection. Suppose
that at some time one of the servers, e.g., S3 becomes inactive.
The status of the cluster is then as follows:
TABLE-US-00002 Port #. 0 1 2 3 4 5 6 7 Server S0 S1 S3 -- S4 -- S5
S6 Bucket 0 1 2 -- 4 4 5
[0055] Notice that the association between servers and buckets has
changed, so that server S4 is now associated with bucket 3, and
server S5 is associated with bucket 4. Now, as there are only five
"alive" severs, the mapping function must produce a value in the
range 0 to 5. If a new connection comes in, and if the mapping
function produces a value 4, then server S6 (not S5) will handle
this connection.
[0056] If a new server S7 is connected to port 5, the number of
servers becomes 7 and the status of the cluster would be:
TABLE-US-00003 Port #. 0 1 2 3 4 5 6 7 Server S0 S1 S2 -- S4 S7 S5
S6 Bucket 0 1 2 3 4 5 6
End of Example I
[0057] Those skilled in the art will realize and understand, upon
reading this description, that the buckets may be renumbered or
reordered in different ways when a server is added to or removed
from the cluster. For example, it may be desirable to give the new
server the bucket number 5 and to leave the other servers as they
were. It should be noted that existing connections are not affected
by server/bucket renumbering because load balancing is only
performed on new connections. Existing (i.e., old) connections
handled entirely in the firewall.
Heartbeat
[0058] Each cluster member 14 includes a so-called heartbeat
processes/mechanism 18. Each heartbeat mechanism 18 (on each
cluster member 14) is a process (or collection of processes) that
performs at least the following tasks: [0059] monitors server
configurations on the cluster; [0060] answers ARP queries for the
configured VIPs; [0061] monitors local state and state of other
cluster members; and [0062] controls local load balancing firewall
configuration.
[0063] The heartbeat monitors the state of VIPs on servers. Each
server may have more than one VIP configured, and the heartbeat
keeps track of each VIP's state separately.
[0064] While described herein as a single mechanism, those skilled
in the art will realize and understand, upon reading this
description, that the various functions of the heartbeat mechanism
can each be considered a separate function or mechanism.
The Heartbeat Mechanism Monitors Server Configuration on the
Cluster
[0065] The heartbeat mechanism 18 on each cluster member/server 14
determines its own state as well as that of each VIP on other
cluster members. (In order to simplify the drawing, not all of the
connections between the various heartbeat mechanisms are shown in
FIG. 1.)
[0066] On each cluster member/server, heartbeat mechanism 18
maintains information about other VIPs in the cluster 10
(preferably all other VIPs). To this end, the heartbeat mechanism
18 builds and maintains a list of VIPs connected to the switch 12,
and then, for each of those VIPs, maintains (and routinely updates)
information. The heartbeat mechanism 18 on each server 14 first
builds a list of network interfaces in the system and obtains
information about IP addresses on these interfaces. The heartbeat
mechanism 18 may, e.g., use, as its main input, a table containing
information about the local cluster and VIPs. In general, an
external process may provide VIP configuration on the local cluster
to the heartbeat process, e.g., in a form of table. Those skilled
in the art will know and understand, upon reading this description
how such a process and table may be defined and configured.
[0067] The heartbeat mechanism 18 considers each VIP in the cluster
10 to be in one of three states, namely "configured", "connecting"
and "connectable". In order to maintain these states, the heartbeat
mechanism 18 obtains a list of VIPs that should be configured on
the cluster 10. Each VIP from the list is preferably cross-checked
against list of IP addresses on all interfaces. If a match is
found, the VIP is marked as "configured". (A VIP is in the
"configured" state--when the VIP is configured on one of the local
(to host) interfaces). For every VIP marked as "configured", the
heartbeat mechanism 18 tries to initiate a TCP connection on a
specified port, e.g., either 80 or 443. As soon as connection to a
VIP is initiated, the VIP is marked as "connecting". If connection
to a VIP is successful, the VIP is marked as "connectable". A VIP's
state is "connecting" when a TCP health check is currently
in-progress; a VIP's state is "connectable" when the most recent
TCP health check succeeded.
[0068] The heartbeat mechanism 18 continuously performs the actions
described above, preferably at fixed, prescribed time
intervals.
[0069] If a VIP changes its state or completely disappears from the
list of IP addresses, a state transition in noted. Servers are
automatically configured (or removed) on (from) loopback clone
interfaces as needed. In a presently preferred implementation, the
heartbeat mechanism takes over the first 100 (lo:0-lo:99) loopback
clone interfaces. If needed, manual loopback interfaces can be
configured starting from lo:100 and up.
The Heartbeat Mechanism Answers ARP Queries for the Configured
VIPS
[0070] Each active heartbeat mechanism 18 continuously listens for
ARP requests. Upon receipt of an ARP request, the heartbeat
mechanism examines request to see if it relates to a VIP that
should be configured on the cluster. If the ARP request does relate
to a VIP, the heartbeat mechanism checks if the VIP is in
"configured" state and if so, the heartbeat mechanism replies with
an ARP reply for that VIP.
[0071] Although multiple heartbeat mechanisms may reply to the same
ARP request, this is not a problem, since they will each return the
same MAC address (MACVIP).
The Heartbeat Mechanism Monitors Local State and State of Other
Cluster Members
[0072] The heartbeat mechanism 18 preferably tries to maintain full
state information for all servers 14 in the cluster 10. State per
cluster preferably includes one or more of: (a) number of cluster
members that should serve traffic for the cluster, (b) number of
cluster members that are serving traffic for the cluster; and (c)
timestamp information. Those skilled in the art will realize and
understand, upon reading this description, that different and/or
other state information may be maintained for the cluster and for
cluster members.
[0073] Each heartbeat mechanism preferably announces its full state
to other cluster members at a prescribed time interval. State
updates are preferably sent to a multicast UDP address which is
shared by all cluster members. (Note: this UDP multicast is not the
same as the VIP multicast discussed above.) The heartbeat mechanism
can also be configured to send multiple unicast UDP messages to
each member of the cluster when performing state announcing.
[0074] Each heartbeat mechanism updates its state upon receiving
state update from other cluster members if the following conditions
are met: the server is present on the receiving cluster member and
the received state is "newer" (per timestamp) than the current
state on receiving cluster member. Since a timestamp is used,
preferably clocks on all cluster members are synchronized.
[0075] At prescribed time intervals a heartbeat mechanism 18
analyzes its state and checks for state transitions. The heartbeat
mechanism checks each server's state and makes sure that it is
fresh. So-called "non-fresh" servers are automatically considered
as "down". Each server's state is compared to its previous state,
and, if different, a state transition is noted.
[0076] Changes to VIP state are made as they detected, based on the
current heartbeat's view of the cluster.
Inter-Cluster Handoff
[0077] As described thus far, server selection has been made within
a cluster by the cluster members at the TCP level. The system does
not require a load balancing switch, thereby reducing the cost.
Instead, as described, the system duplicates incoming
(client-to-cluster) traffic to all servers in the cluster and lets
each server decide if it is to deal with particular part of the
incoming traffic. All servers in the cluster communicate with each
other and decide on an individual server's health.
[0078] Another level of server selection--within a cluster--is also
provided, as a result of which an initially-selected server
(selected as described above) may pass on (or attempt to pass on)
responsibility for a particular connection to another cluster
member. For example, if one server in a cluster has already handled
a particular request for a certain resource, that server may have
that resource cached. The server with the already-cached copy of
the resource may then be a better choice than another server in the
cluster to process a request.
[0079] Accordingly, in some cases, after receiving a request from a
client for a certain resource (after a server has been selected and
the TCP connection has been established, as described above), the
server may ascertain whether it is responsible for handling/serving
the resource, and, if not, the previously-selected server may
notify (or provide a notification) to another cluster member that
is responsible for handling the resource (e.g., another cluster
member that already has a copy of the requested resource). The
notification may include a hand-off request to so that another
cluster member responsible for the resource can server the resource
itself. Or, alternatively, the notification may include a request
for a copy of the resource (e.g., via a peer-fill request) from
another cluster member responsible for the resource (i.e., that
already has a copy of the requested resource).
[0080] The cluster member responsible for (handling) the requested
resource may process the notification from the previously or
originally selected server in a number of ways. For instance, a
cluster member that has previously served the requested resource
(or that is `responsible` for handling the request, or already has
a copy of the requested resource) may determine whether to accept
or reject a hand-off request (or a peer-fill request) from the
previously or originally selected server. For example, the other
cluster member may decide to accept or reject the hand-off request
(or peer-fill request) based on various attributes of the requested
resource such as, but not limited to, the size and popularity of
the requested resource.
[0081] In one embodiment, the responsible server accepts a hand-off
request (or rejects a peer-fill request) if the size of the request
resource exceeds a threshold value. This step is advantageous
because copying a large resource to the previously selected server
is inefficient and would not be a worthwhile expenditure of system
and network resources. If, on the other hand, the size of the
requested resource is small (i.e., does not exceed a size
threshold), then it may be worthwhile to reject the hand-off
request (or accept the peer-fill request) and provide a copy of the
requested resource to the previously selected sever so that the
previously selected server can handle the request.
[0082] According to another example embodiment, if it determined
that the requested resource is popular (i.e., exceeds a popularity
threshold), then the responsible server may reject the hand-off
request (or accept/honor the peer-fill request) and (indirectly)
force the previously selected server to obtain and serve the
requested resource (or simply provide a copy of the requested
resource to the previously selected server). Since the resource is
popular and, thus, likely to continue to be requested frequently,
it would be beneficial for other servers (i.e., the previously
selected server) to have a copy of the requested resource so that
the requested "popular" resource can be served more efficiently.
For example, in addition to sending a hand-off rejection message,
the responsible server may also provide a copy of the requested
resource to the previously selected server (or the previously
selected server may also obtain a copy of the requested resource
from other sources, such as other peers, upstream servers,
etc.).
[0083] As used herein, a "resource" may be any kind of resource,
including, without limitation static and dynamic: video content,
audio content, text, image content, web pages, Hypertext Markup
Language (HTML) files, XML files, files in a markup language,
documents, hypertext documents, data files, and embedded
resources.
[0084] Once a TCP/IP connection is made between two machines (e.g.,
client 19 and a particular cluster member, server 14-k (for some
value of k)), the server 14-k may receive a request from the client
19, e.g., for a resource. For example, the server 14-k may receive
an HTTP request (e.g., an HTTP GET request) from client 19. Such a
request generally includes a URL along with various HTTP headers
(e.g., a host header, etc.). The selected server 14-k now
determines whether it is responsible to handle this request or
whether the request should be passed on to a different cluster
member. To make this determination, the selected server 14-k
considers the request itself and applies a second given function to
at least some of the information used to make the request (e.g., to
the URL and/or headers in the request).
[0085] This second function essentially partitions the request
space (e.g., the URL space) so as to determine whether the selected
server is, in fact, responsible to for this particular request. If
the server determines that it is responsible for the request, it
continues processing the request. If not, the server hands-off the
request (as described below) on to another cluster member (e.g.,
server 14-p) that is responsible for the request. Having
successfully passed off the request, the cluster member, server
14-k, updates its firewall to reject packets associated with the
connection. The responsible cluster member (server 14-p)
correspondingly updates its firewall to accept packets associated
with this connection.
[0086] For the sake of this discussion, the function used to
partition the requests is referred to as a partition function. The
partition function may be a hash function or the like. In some
cases the partition function may take into account the nature or
type of request or resource requested. For example, certain cluster
members may be allocated to certain types of requests (e.g.,
movies, software applications, etc.). The partition function
applied to the URL (and/or other information) can be used to
implement a degree of policy based load mapping.
[0087] Exemplary partition functions are:
Partition(URL,m).fwdarw.{1 . . . m}
Partition(URL,host header,m).fwdarw.{1 . . . m}
Partition(URL,HTTP headers,m).fwdarw.{1 . . . m} [0088] where
Partition (params, m) is implemented as, e.g.,
[0088] hash(params)modulo m [0089] where m is the number of active
servers in the cluster.
[0090] Those skilled in the art will realize and understand, upon
reading this description, that different and or other parameters
may be used in the Partition function. Further, not all parts of a
parameter need be used. For example, if the URL is a parameter, the
function may choose to use only a part of the URL (e.g., the
hostname).
[0091] Since accounting and other information may be included in
HTTP headers and/or URLs, such information may be used by the
partition function. For example, a cluster may comprise a number of
non-homogenous servers. Certain requests may be directed to certain
cluster servers based on server capability (e.g., speed) or based
on arrangements with customers.
[0092] In order to hand off a request to another server within its
cluster, a server must be able to completely move an individual
established TCP connection from one server to another in the same
cluster. The following scenario, with references to FIGS. 2-4,
describe this operation of the system. As shown in the FIG. 2, the
cluster includes two servers: server A and server B. Each of the
servers runs a web cache, listening on a shared VIP (and port,
e.g., port 80). Remote clients make incoming TCP connections to the
VIP and port (as described above).
[0093] Using the TCP-level load balancing described above, assume
that server A is initially selected to accept a particular TCP
connection from a client (at S30 in FIG. 3). Server A accepts the
connection from the client and waits for the HTTP request from the
client. Using information from the HTTP request (e.g., the URL and
one or more HTTP headers) server A decides to hand the request off
to the server B. That is, the selected server (server A in this
example) ascertains (using the partition function described above)
whether it is the server responsible for the request (at S31). If
the originally-selected server is responsible for the request (at
S32), then it handles the request (at S33), otherwise it hands off
(or tries to hand off) the request to the responsible cluster
member (server B in this example) (at S34). If the handoff is
determined to be successful (at S35), then the server responsible
for the request (Server B in the example) handles the request (at
S36), otherwise the originally selected server (Server A) handles
the request (at S37).
[0094] The hand-off process (S34) takes place as follows (with
reference to FIG. 4) (for the purposes of this discussion, assume
that server A hands off to server B):
[0095] First the originally-selected server (Server A) freezes the
TCP connection from the client (at S40). The selected server
(Server A) then takes a snapshot of the frozen TCP connection (at
S41), storing required information about the connection. The
originally-selected server (Server A) then sends the snapshot of
the frozen TCP connection to the responsible server (server B),
preferably using a side communication channel to the responsible
server (at S42).
[0096] The responsible server (Server B) receives the snapshot of
the frozen TCP connection from the originally-selected server
(Server A) (at S43). Using the snapshot of the frozen TCP
connection, the responsible server (Server B) attempts to clone the
TCP connection to the remote client (at S44). If the connection was
cloned successfully, the responsible server (server B) sends
acknowledgement to the originally-selected server (Server A),
preferably using the side communication channel to the server A (at
S45).
[0097] Upon receipt of the acknowledgement, the originally-selected
server (Server A) closes the frozen TCP connection to the client
(at S46).
[0098] The responsible server (Server B) then thaws the frozen
(clone) TCP connection to the client (at S47).
[0099] With the handoff successful, the responsible server (Server
B) continues to process incoming HTTP request from the client (at
52 in FIG. 4).
[0100] The accepting server may fail to clone connection or may
refuse to satisfy handoff request. In these cases a negative
acknowledgment will be sent and originating (handoff) server will
continue to process original request. Should the responsible server
(Server B) decline (or fail to satisfy) the handoff request from
the originally-selected server (Server A), server A may thaw the
TCP connection and continue to serve it locally.
[0101] A responsible server generally should not decline a handoff
request or a request to take over a connection. However, a
responsible server may have to decline a request, for example if
its software is being shutdown. Note, too that two or more servers
in the same cluster may be responsible for the same content, and
may provide a degree of redundancy in content (to reduce fills from
the origin server) and also to handle a so-called "flash crowd"
when a certain piece of content becomes very popular for a
relatively short period time.
[0102] When a handoff is successful, the responsible server must
update its firewall to accept packets relating to that connection
(and the server that handed off the connection must update its
firewall to no longer accept such packets).
[0103] It should be apparent that only the server that is actually
handling the connection will invoke the partition function. The
other servers do not generally have the information required (e.g.,
the URL) to make the required decision.
[0104] The server making the handoff may provide the responsible
server with information about the request (e.g., the type of
request, the URL, the headers, etc.). In this way the responsible
server may have sufficient information to satisfy the request.
Example II
[0105] By way of example, and without limitation, consider a
cluster with 8 ports and with 7 active servers connected to those
ports as shown in the following table:
TABLE-US-00004 Port #. 0 1 2 3 4 5 6 7 Server S0 S1 S2 S3 S4 -- S5
S6 Bucket 0 1 2 3 4 5 6
[0106] In this case, the number of active servers, m, is 7, there
are seven buckets (numbered 0 to 6), and so the mapping function
should produce a number in the range 0 to 6. Suppose, for the sake
of this example, that the mapping function is:
MAP(source IP,destination IP,destination port,m)=hash(source
IP,destination IP,destination port)modulo m
[0107] If a connection request comes in from IP address
123.156.189.123, for the VIP (1.0.0.1) on port 80. Each server runs
the mapping function hash (123.156.189.123, 1.0.0.1, 80) modulo
7
[0108] Suppose that this mapping produces a value of 4 then server
S4 (which corresponds to bucket 4) is selected at the TCP level to
handle the connection. Server S4 and the client then establish
their connection and the client then sends an HTTP request (e.g., a
GET request with a URL (URL1) and header information).
[0109] Server S4 invokes the partition function:
Partition(URL1,host header,7)
[0110] Note that the partition function can use the same bucket
association as the mapping function or it may use a different
association. For example, if the partition function is implementing
policy-based or capacity based distribution, then the partition
function may need a separate bucket association. For this example,
assume that the partition function uses the same bucket association
as the mapping function.
[0111] Suppose that this invocation of the partition function
returns a value of 6. This means that server S6 (associated with
bucket no. 6) should handle this connection instead of the
initially-selected server S4. So server S4 tries to hand off the
connection to server S6.
[0112] Server S4 freezes the TCP connection from the client (at S40
in FIG. 4) and then takes a snapshot of the frozen TCP connection,
storing required information about the connection (at S41). Server
S4 sends the snapshot of the frozen TCP connection to Server S6,
preferably using a side communication channel (at S42). Server S6
receives the snapshot of the frozen TCP connection from Server S4
(at S43). Using the snapshot of the frozen TCP connection, Server
S6 attempts to clone the TCP connection to the remote client (at
S44). If the connection is successfully cloned, then server S6
sends an acknowledgement to Server S4, preferably using the side
communication channel (at S45). Upon receipt of the
acknowledgement, Server S4 closes the frozen TCP connection to the
client (at S46). Server S6 then thaws the frozen (clone) TCP
connection to the client (at S47). With the handoff successful,
Server S6 continues to process incoming HTTP request from the
client.
[0113] Suppose now that another connection request comes in, this
time from IP address 123.156.111.123, for the VIP (1.0.0.1) on port
80. Each server runs the mapping function:
hash(123.156.111.123,1.0.0.1,80)modulo 7
[0114] Suppose that the result of this function is 6 which
corresponds to server S6. S6 connects with the client and the
client then sends an HTTP GET request with a URL (URL1--the same as
in the earlier request) and header information.
[0115] Server S6 invokes the partition function:
Partition(URL1,host header,7)
[0116] Again the partition function returns the value 6. However,
in this case the server responsible for the request is the one
already handling the request, and so no handoff is needed (i.e.,
the check at S32 will return "YES"). Note that since server S6 has
already served the resource associated with URL1, it may still have
that resource cached.
End of Example II
[0117] Note that the number of servers connected to the switch
could be greater than the number of servers responsible for the
VIP. For example, a cluster may be configured with 20 servers
connected to the same switch, 10 servers serving one VIP and
another 10 servers serving another VIP. In this case the heartbeat
assists in load balancing for two VIPs, and each VIP will be load
balanced across 10 servers.
[0118] As shown in FIG. 5, a collection of load-balancing clusters
10-1, 10-2, . . . , 10-p, may be combined. Each cluster 10-j has
one or more corresponding VIPs (VIP-j), so that requests for a
server at the IP address VIP-k (for some value of k) will be
directed (by router 110) to the appropriate cluster for handling by
one of the cluster members. The router 110 may be, e.g., a load
balancing router.
[0119] A client 19 may request a resource and be directed by a
server selector system (e.g., DNS or the like) to a cluster. The
server selector returns an IP address that happens to be a VIP
address. The client then requests the resource from the VIP and, as
described above, is connected (during a TCP connection) to a
particular cluster member to handle the request.
[0120] If the cluster implements the partitioning function, then
the connection may be handed off to another cluster member.
[0121] FIG. 6 (6A and 6B) is a flowchart (600-1 and 600-2) of
processing steps associated with server interactions.
[0122] In step 605, the cluster (i.e., via a switch) obtains a
connection request to connect to a server associated with the
virtual IP address (i.e., any server sitting behind the switch
associated with a virtual IP address).
[0123] In step 610, the cluster (i.e., via the switch) provides the
connection request to each server connected to the switch.
[0124] In step 615, at least one of the plurality of servers
connected to the switch determines which of the plurality of
servers should handle the connection. Such a determination can be
based, for example, on a given function of information used to
request the connection.
[0125] In step 620, if the server that is determined to handle the
request does not have a copy of the requested resource, that server
then requests to hand-off the connection (i.e., TCP connection) to
at least one other of the plurality of servers that does have a
copy of the requested resource. Note that the server may request a
copy of the requested resource (e.g., via a peer-fill request) from
another server that has a copy of the resource instead of sending a
hand-off request.
[0126] In step 625, the server that has a copy of the requested
resource determines whether to accept or reject the hand-off
request (or reject or accept the peer-fill request) from the server
that was originally determined to handle the connection/request.
This determination can be based, for example, on the size of the
requested resource, the popularity of the requested resource, as
well as other attributes that are suitable for determining whether
or not a TCP hand-off should occur in a server cluster in response
to a request for certain resources.
[0127] In step 630, the server that has the copy of the requested
resource accepts the hand-off request (or rejects the peer-fill
request) if the size of the requested resource value exceeds a
threshold value. In this example embodiment, if the size of the
requested resource is determined to be too large (i.e., exceeds a
threshold value) for expending precious system and network resource
(i.e., by providing intra-cluster copies of resources, for example,
one server sending a copy of a resource to another server in the
cluster), then the server with the requested resource will handle
the request itself (i.e., serve the requested resources, and, for
example, not honor the peer-fill request).
[0128] In step 635, the server that has the copy of the requested
resource accepts the hand-off request (or rejects the peer-fill
request) if the popularity of the requested resource does not
exceed a popularity threshold value. In other words, if it
determined that the requested content is not popular (i.e., the
number of times the particular resource has been requested during a
retrospective time period does not exceed a threshold value), then
the server with the copy of the request resource handles the
connection and serves the resource (and, for example, does not
honor the peer-fill request). Since the resource is not yet deemed
popular, it is likely that the resource will not be requested as
often and therefore is would not be efficient to transfer copies of
the resource to other servers in the cluster.
[0129] In step 640, the server that has the copy of the requested
resource rejects the hand-off request (or accepts/honors the
peer-fill request if a copy of the resource is available) if the
popularity of the requested resource exceeds the popularity
threshold value. In this example circumstance, since it is
determined that the requested content is popular, then it further
behooves the cluster to have copies of the requested resource on
other servers in the cluster to handle the possibility of more
requests for the popular resource. Thus, instead of accepting the
hand-off request, the server with the copy of the requested
resource rejects the request, which, in one embodiment, forces the
requesting server to obtain and serve the requested resource itself
(and, thus, maintain a copy of the popular resource, for example,
by honoring the peer-fill request and thus providing a copy of the
requested resource).
[0130] In step 645, the server that has the copy of the requested
resource rejects the hand-off request (or accepts/honors the
peer-fill request if a copy of the resource is available) if the
popularity of the requested resource exceeds the popularity
threshold value and the size of the requested resource exceeds the
threshold size value. This particular step elucidates the
significance of popular content. Even if the size of the requested
resource is deemed to large to send an intra-cluster copy from one
server to another server within the same cluster (i.e., in light of
the expenditure to system and network resources within the
cluster), the popularity of the content may still make it more
efficient in the long run to distribute a copy (or copies) of the
requested resource throughout the cluster in anticipation of more
requests for the popular content at the cluster. For example, one
way to distribute copies of the requested resource is to reject the
hand-off request and (either directly or indirectly) force the
originally-selected server to handle the connection and ultimately
serve the requested resource.
[0131] FIG. 7 is a flowchart 700 of processing steps associated
with server interactions.
[0132] In step 705, a connection request to connect to a server
associated with the IP address is received (e.g., at a cluster
comprising a switch and plurality of server connected thereto via
one or more ports of the switch).
[0133] In step 710, a determination is made as to which of the
plurality servers is to handle the connection (e.g., via a hash
function).
[0134] In step 720, if a first server of the plurality of servers
is determined to be the server to handle the connection (e.g., via
the hash function), and the first server does not have a copy of
the requested resource, the first server provides a notification to
a second server of the plurality of servers that does have a copy
of the requested resource. In one example embodiment, the
notification indicates that the first server does not have a copy
of the requested resource. Alternatively, the notification can
include a hand-off request to hand-off the connection to another
server (e.g., the second server in this step), and/or a peer-fill
request that requests a copy of the requested resource from another
server (e.g., the second server in this step).
[0135] In step 725, in response to receiving the notification from
the first server, the second sever determines whether to: i)
provide a copy of the requested resource to said server (e.g.,
reject a hand-off request or accept a peer-fill request if a copy
of the requested resource is available), or ii) request the server
to handoff the connection to the second server so that the second
server can serve the requested resource (e.g., accept a hand-off
request or reject a peer-fill request). For example, in one
embodiment this determining may be based on an attribute of the
requested resource (e.g., size, popularity, etc.).
[0136] Although aspects of this invention have been described with
reference to a particular system, the present invention operates on
any computer system and can be implemented in software, hardware or
any combination thereof. When implemented fully or partially in
software, the invention can reside, permanently or temporarily, on
any memory or storage medium, including but not limited to a RAM, a
ROM, a disk, an ASIC, a PROM and the like.
[0137] While certain configurations of structures have been
illustrated for the purposes of presenting the basic structures of
the present invention, one of ordinary skill in the art will
appreciate that other variations are possible which would still
fall within the scope of the appended claims. While the invention
has been described in connection with what is presently considered
to be the most practical and preferred embodiment, it is to be
understood that the invention is not to be limited to the disclosed
embodiment, but on the contrary, is intended to cover various
modifications and equivalent arrangements included within the
spirit and scope of the appended claims.
* * * * *